DESCRIPTION The ctime(), gmtime() and localtime()
functions all take an argument of data type time_t which
represents calendar time. When interpreted as an absolute
time value, it represents the number of seconds elapsed
since 00:00:00 on January 1, 1970, Coordinated Universal
Time (UTC).

The asctime() and mktime() functions both take an
argument representing broken-down time which is a
representation separated into year, month, day, etc.

Broken-down time is stored in the structure tm which is
defined in <time.h> as follows:

tm_sec The number of seconds after the minute, normally
in the range 0 to 59, but can be up to 60 to allow for leap
seconds.

tm_min The number of minutes after the hour, in the
range 0 to 59.

tm_hour The number of hours past midnight, in the range
0 to 23.

tm_mday The day of the month, in the range 1 to 31.

tm_mon The number of months since January, in the range
0 to 11.

tm_year The number of years since 1900.

tm_wday The number of days since Sunday, in the range 0
to 6.

tm_yday The number of days since January 1, in the range
0 to 365.

tm_isdst A flag that indicates whether daylight saving
time is in effect at the time described. The value is
positive if day- light saving time is in effect, zero if it
is not, and nega- tive if the information is not
available.

The call ctime(t) is equivalent to
asctime(localtime(t)). It converts the calendar time t into
a null-terminated string of the form

"Wed Jun 30 21:49:08 19930

The abbreviations for the days of the week are
"Sun", "Mon", "Tue",
"Wed", "Thu", "Fri", and
"Sat". The abbreviations for the months are
"Jan", "Feb", "Mar",
"Apr", "May", "Jun",
"Jul", "Aug", "Sep",
"Oct", "Nov", and "Dec". The
return value points to a statically allocated string which
might be overwritten by subsequent calls to any of the date
and time functions. The function also sets the external
variables tzname, timezone, and daylight (see tzset(3)) with
information about the current timezone. The reentrant
version ctime_r() does the same, but stores the string in a
user-supplied buffer which should have room for at least 26
bytes. It need not set tzname, timezone, and daylight.

The gmtime() function converts the calendar time timep
to broken-down time representation, expressed in Coordinated
Universal Time (UTC). It may return NULL when the year does
not fit into an integer. The return value points to a
statically allocated struct which might be overwrit- ten by
subsequent calls to any of the date and time functions. The
gmtime_r() function does the same, but stores the data in a
user-sup- plied struct.

The localtime() function converts the calendar time
timep to broken- time representation, expressed relative to
the user s specified time- zone. The function acts as if it
called tzset(3) and sets the external variables tzname with
information about the current timezone, timezone with the
difference between Coordinated Universal Time (UTC) and
local standard time in seconds, and daylight to a non-zero
value if daylight savings time rules apply during some part
of the year. The return value points to a statically
allocated struct which might be overwrit- ten by subsequent
calls to any of the date and time functions. The
localtime_r() function does the same, but stores the data in
a user- supplied struct. It need not set tzname, timezone,
and daylight.

The asctime() function converts the broken-down time
value tm into a null-terminated string with the same format
as ctime(). The return value points to a statically
allocated string which might be overwrit- ten by subsequent
calls to any of the date and time functions. The asctime_r()
function does the same, but stores the string in a user-
supplied buffer which should have room for at least 26
bytes.

The mktime() function converts a broken-down time
structure, expressed as local time, to calendar time
representation. The function ignores the values supplied by
the caller in the tm_wday and tm_yday fields. The value
specified in the tm_isdst field informs mktime() whether or
not daylight saving time (DST) is in effect for the time
supplied in the tm structure: a positive value means DST is
in effect; zero means that DST is not in effect; and a
negative value means that mktime() should (use timezone
information and system databases to) attempt to determine
whether DST is in effect at the specified time.

The mktime() function modifies the fields of the tm
structure as fol- lows: tm_wday and tm_yday are set to
values determined from the con- tents of the other fields;
if structure members are outside their valid interval, they
will be normalized (so that, for example, 40 October is
changed into 9 November); tm_isdst is set (regardless of its
initial value) to a positive value or to 0, respectively, to
indicate whether DST is or is not in effect at the specified
time. Calling mktime() also sets the external variable
tzname with information about the cur- rent timezone.

If the specified broken-down time cannot be represented
as calendar time (seconds since the Epoch), mktime() returns
a value of (time_t) -1 and does not alter the members of the
broken-down time structure.

RETURN VALUE Each of these functions returns the value
described, or NULL (-1 in case of mktime()) in case an error
was detected.

NOTES The four functions asctime(), ctime(), gmtime()
and localtime() return a pointer to static data and hence
are not thread-safe. Thread-safe versions asctime_r(),
ctime_r(), gmtime_r() and localtime_r() are spec- ified by
SUSv2, and available since libc 5.2.5.

POSIX.1-2001 says: "The asctime(), ctime(),
gmtime(), and localtime() functions shall return values in
one of two static objects: a broken- down time structure and
an array of type char. Execution of any of the functions may
overwrite the information returned in either of these
objects by any of the other functions." This can occur
in the glibc implementation.

In many implementations, including glibc, a 0 in tm_mday
is interpreted as meaning the last day of the preceding
month.

defined when _BSD_SOURCE was set before including
<time.h>. This is a BSD extension, present in
4.3BSD-Reno.

According to POSIX.1-2004, localtime() is required to
behave as though tzset() was called, while localtime_r()
does not have this requirement. For portable code tzset()
should be called before localtime_r().